1. Field of the Invention
This invention relates to a cold rolled steel sheet useful for automobiles and a method of producing the same, and more particularly to an extra-low carbon cold rolled steel sheet having an improved spot weldability without damaging excellent formability.
2. Related Art Statement
Cold rolled steel sheets having improved formabilities, particularly deep drawability are mainly applied to inner and outer panels for automobiles. Therefore, there have hitherto been made studies so as to provide optimum conditions satisfying mechanical properties required for steel sheets when the sheet is press-formed into parts for automobiles. Particularly, the steel sheets for automobiles should be adapted to a great variety of designs, so that the improvement of r-value corresponding to the deep drawability or the reduction of yield stress and improvement of work hardenability from a viewpoint of shape freezability are also important. For this end, extra-low carbon steels having a carbon content reduced to a level of about 10 ppm becomes adopted lately.
The production technique of extra-low carbon steel sheets for deep drawing developed from the above viewpoints is disclosed, for example, in Japanese Patent laid open No. 59-193,221 or in the specification of Japanese Patent Application No. 61-219,803 previously filed by the inventors.
In these techniques, however, spot weldability and properties of weld portion are not considered as important properties in addition to the formability. Particularly, the extra-low carbon steel is generally poor in spot weldability as compared with low carbon steels.
The spot welding operation is an indispensable factor in the assembling work of parts formed by pressing or other process. Therefore, the operability of such a spot welding as well as mechanical properties of weld portion are important together with the formability in view of the evaluation on total properties of the steel sheet.
Moreover, the spot weldability is barely reported in Japanese Patent laid open No. 61-110,757, but the control of the thickness of surface oxide film disclosed therein is very difficult to be applied to industrial production and is hardly practical.
In general, it has been well-known to improve elongation (El) and the Lankford value (r-value) and reduce Y.S. (low Y.R.) from a viewpoint of formabilities, particularly deep drawability and shape freezability in the press forming, which have been realized by extremely reducing the carbon content of steel sheet. However, when such an extra-low carbon steel is subjected to spot welding, the strengths are poor as compared with those of the conventional low carbon steel and the proper welding condition is shifted toward a high welding current side as compared with the conventional low carbon steel as shown in FIG. 20. The result is that a new problem arises in that the consumption of a spot welder becomes faster.